1. Field of the Invention
The present invention generally relates to a circuit utilized for signal transmission, and more particularly, to a signal mixing method and signal mixing circuit for eliminating harmonic mixing signals
2. Description of the Prior Art
The harmonic mixing problem is often seen in many wideband signal transmission applications. Please refer to FIG. 1 and FIG. 2. FIG. 1 is a diagram of a conventional mixer. As shown in FIG. 1, the mixing unit 100 of the mixer receives an input signal 101 and an oscillation signal 102, provided by the mixer, to generate a mixed output signal 103. Taking the digital TV for example, the entire frequency band is 48 MHz-860 MHz, where each channel bandwidth is 6 MHz, 7 MHz, or 8 MHz. If the signal carried in the first channel (˜52 MHz) is fetched through the direct down conversion, not only the wanted signal having the frequency (˜52 MHz) is obtained, but also signals having the harmonic frequencies 2Fo, 3Fo, . . . , nFo. As shown in FIG. 2, this causes the output signal obtained to include not only the first channel signal as expected but also the harmonics of the first channel signal. This is due to the fact that the mixing unit 100 is usually not an ideal multiplier and harmonics are included in the oscillation signal provided. For example, in the case of an oscillation square wave signal, the Fourier series of the square wave can be represented as a plurality of sine waves with components having different frequencies. Therefore, in the frequency domain, unlimited harmonics can be observed, and when the harmonics fall within the frequency band of interest, the above-mentioned harmonic mixing problem occurs.
To solve the harmonic mixing problem, one conventional way is to apply an up conversion and then a down conversion. That is, the frequency Fo of the oscillation signal is set to be at a high frequency such that the harmonic frequencies 2Fo, 3Fo, . . . , nFo do not fall within the frequency band of interest. However, this solution requires additional mixing units and generally utilizes an external surface acoustic wave (SAW) filter and therefore, and is not a cost effective solution.